In a known sensor arrangement for reflection light barriers of this type, two LED chips and one photodiode chip are arranged on a circuit board. The LED chip provided for the transmission of the reference light beam directly irradiates the adjacent photodiode, with a large part of the light being radiated into the side surface of the photodiode due to the position of both chips on one plane. The other LED chip provided for the transmission of the useful light beam, in contrast, only radiates upwardly, since it is located in a separate part of the housing. If a reflecting object is located in its beam cone, the reflected light is incident on the photodiode from above.
On the irradiation of a photodiode with a light pulse, the photo current resulting from the irradiation of the photodiode follows the light pulse with a specific delay which is quantified by the rise time and the decay time of the photo current. These times generally depend both on the electrical parameters of the photodiode, above all on its capacity, and on its external electrical wiring, substantially on the load resistance. There is furthermore, however, a clear dependence on the direction of the optical radiation. If the light is not incident onto the semiconductor chip primarily from above, but penetrates into the chip through the side surfaces, a considerable increase in the rise time and decay time occurs. The photodiode therefore becomes slower. The reason for this is probably the following: with lateral radiation, a large portion of the production of free charge carrier pairs caused by absorption of the light takes place in the substrate of the chip. Before these charge carrier pairs can contribute to an electrical current, they must first diffuse into the region of the p-n junction, which results in a delay.
In the specific application, both LED chips are operated in an alternately pulsed manner, whereby the sensitivity to ambient light of the reflection light barrier can be eliminated with the help of an electronic circuit. The different time behavior for the light radiation onto the photodiode from the side or from above causes large problems for the evaluation electronics in this process.